The present invention relates to a technique for providing higher reliability of cell switching function of a switching system or a transmission system included in a communication network using an asynchronous transfer mode (hereafter abbreviated to ATM), and in particular to a system switching method, and apparatus, causing no loss o information at the time of system switching in a duplex operation system.
In conventional communication networks using a synchronous transfer mode (hereafter abbreviated to STM), it is possible to make the operation of a primary system of a switch portion synchronize with that of a standby system thereof so that loss of information at the time of system switching may be avoided. That is to say, loss of a bit due to switching is prevented provided that switch portions of both systems are synchronized with the precision of hit phase of a digital signal.
Cell switches in ATM communication networks switch information by taking data hits having a fixed length called a cell as the unit and routing information for switching is added to the header of the cell. Unlike the conventional STM switch in which switching is performed according to the bit position, the bit position cannot become information for switching.
In cell switches of an ATM, therefore, synchronizing the primary system and the standby system for the purpose of system switching impairs the advantage of the ATM in that switching is performed by labels of cell headers, and hence it is not effective means. Further, in the ATM, cells are buffered within the switch and hence it is difficult to control that Phase. It is thus difficult to prevent loss of information by means of simple switching as in the STM.
By the way, JP-A-63-70698 discloses a time division switching system of STM allowing continuation of communication without loss of signal when channels are rearranged during communication as occasion demands. However, it does not solve the above described problem.